1. Field of the Invention
The present invention generally relates to a switching power supply circuit, and more particularly to a high-frequency converter arranged to operate at a high duty ratio.
2. Description of the Prior Art
Various types of switching power supply circuits have been developed as, for instance, forward converters, push-pull converters and bridge converters.
As is well known in the art, forward converters have particular advantages in that no asymmetrical flux swings, or dissymmetry phenomenon occurs, as compared with push-pull converters and bridge converters. In push-pull converters and bridge converters, generally, the asymmetrical flux swing phenomenon occurs during transition periods, in load fluctuations or in aging effects of switching transistors, because excited magnetic fluxs cannot be reset during turn-off periods of switching transistors.
FIG. 1 shows a typical double ended DC-to-DC forward converter.
A basic operation of the forward converter is as follows. An electric power is transferred from a primary circuit of a switching transformer to a secondary circuit thereof during turn-on periods of switching transistors. These switching transistors are simply represented by switch contacts. An exciting energy of the switching transformer is reset during turn-off periods of the switching transistors.
The magnetic excitation in the switching transformer will be simply explained. When a voltage E.sub.i of a DC power supply 11 is applied to the switching transformer having the primary winding of a turn number n.sub.1 for only a time period T.sub.on, a magnetic flux .phi..sub.set generated in the core of the switching transformer is given by the following equation (1). ##EQU1##
That is, the magnetic flux .phi..sub.set is given by the product of applied voltage E.sub.i and period "T.sub.on " for applying this voltage and is equivalent to an electric power which passes through the switching transformer. In other words, a total amount of magnetic flux .phi..sub.set is not influenced by the electric power which is transferred to the secondary winding.
A typical magnetic material has a limit value which is defined by the maximum magnetic flux density. This flux density is equal to a value which is obtained by dividing the produced magnetic flux by the effective sectional area of the core, and is namely, the magnetic flux per unit area. If the magnetic material is excited to a value in excess of the limit value, it will be saturated and therefore the permeability of the magnetic material will promptly decrease. Thus, the inductance of the primary winding is rapidly approximated to the inductance of the air-core coil, so that the switching transformer utilizing such a magnetic material will fail to perform its transformer function.
Therefore, the magnetic flux excited during the preceding turn-on (T.sub.on) period needs to be firmly reset during the turn-off (T.sub.off) period of a switching transistor.
For a better understanding of the foregoing fundamental operation, the operation of the double ended type DC-to-DC forward converter in FIG. 1 will now be explained hereinbelow.
The conventional DC-to-DC forward converter includes first and second switching transistors 1A and 1B, a switching transformer 8 having a primary winding 3 and a secondary winding 5; and first and second feedback diodes 2A and 2B. The first and second switching transistors 1A and 1B are series-connected to the primary winding 3 of the switching transformer 8. The first and second feedback diodes 2A and 2B are connected between the DC power supply 11 and the corresponding switching transistors 1A and 1B in such a manner that the reverse current can be fed back, or returned to DC power supply 11 namely to the forward converter during the turn-off periods of the switching transistors 1A and 1B. To the secondary winding 5, a rectifier and smoothing filter circuit having a rectifier diode 4 is connected to derive a DC output. The turn number of the primary winding 3 is selected to be n1, whereas that of the secondary winding 5 is n2.
The fundamental operation of the forward converter in the double ended DC-to-DC forward converter having the above circuit arrangement will now be explained.
In the converter in FIG. 1, when both switching transistors 1A and 1B are turned off, the counter electromotive force which has been generated in primary winding 3 of switching transformer 8, is returned to DC power supply E.sub.i diodes 2A and 2B, so that the voltage across primary winding 3 is clamped to the power source voltage and the excited magnetic flux is reset.
A magnetic flux .phi..sub.res to be reset is given by the following equation (2). ##EQU2##
Therefore, the condition regarding the magnetic excitation necessary to make the converter in FIG. 1 operative will be given by: EQU .phi..sub.set .ltoreq..phi..sub.res ( 3)
By substituting the equations (1) and (2) for the inequality (3), we have ##EQU3##
The theoretical limit value of the duty ratio ##EQU4## since T.sub.on .ltoreq.T.sub.off. However, the theoretical limit value is practically reduced to a value of up to 30 to 40% due to safety reasons.
The single transistor converter shown in FIG. 2 is of the type in which a switching transformer 9 has a third winding 6 for resetting the magnetic excitation. In the above prior art converter, the magnetic flux excited is returned to DC power source E.sub.i through a diode 2C.
Magnetic flux .phi..sub.res to be reset in this case is given by the following equation (5). ##EQU5## As can be understood from this equation, when a turn number n.sub.3 of third winding 6 is reduced, the magnetic flux can be sufficiently reset even if the turn-off period T.sub.off is reduced.
As an example, assuming that a turn ratio is set to n.sub.3 =0.5n.sub.1, the limit value of the duty ratio will amount to approximately 67%.
However, since primary and third windings 3 and 6 are magnetically coupled to each other in switching transformer 9, when diode 2C is turned on and the voltage across third winding 6 is clamped to the power source voltage of -E.sub.i while switching transistor 1 is turned off, a great counter electromotive force given below is generated in primary winding 3. ##EQU6## This counter electromotive force is superimposed on voltage E.sub.i of DC power supply 11 and applied to a switching transistor 1, so that the total voltage inevitably becomes: ##EQU7##
However, as described above, the conventional forward converters in FIGS. 1 and 2 have an advantage such that the excited energy can be reset during the turn-off period of the switching transistor. In other words, the forward converter has an advantage such that there is no saturation of the core of the switching transformer by an asymmetrical flux swing which causes a problem in the conventional push-pull type or bridge type converter. Nevertheless, there are drawbacks such that the duty ratio cannot exceed 50% and the efficiency of the switching transformer cannot be effectively increased.
In addition, such a forward converter is generally operated at a high input voltage, e.g., at a DC voltage of 200 to 370 V and further performs the switching operation at a high frequency on the order of, e.g., 100 kHz. Therefore, if the forward converters as shown in FIGS. 1 and 2 are made operative by use of commercially available switching transistors, the duty ratio cannot be sufficiently great, and there is the risk such that these transistors are broken down by the counter electromotive voltage, which is about three times as high as the power source voltage as mentioned above.
In addition, there is also another problem such that if the commercially available switching transistors of reasonable prices are employed, the design of the circuit elements and operating conditions will be limited, due to the foregoing problems.
It is, therefore, an object of the present invention to provide a switching power supply circuit in which the duty ratio can exceed 50%, the asymmetrical flux swing (DC excitation) phenomenon does not occur, and the magnetic flux excited during the switching-on periods can be sufficiently reset.
It is another object of the invention to provide a switching power supply circuit which can perform the switching operation at a high source voltage and at a high frequency.